1. Field of the Invention
This invention relates to a charging apparatus for use in an appliance accommodating a secondary battery therein or a charging apparatus for charging a secondary battery, and more particularly, to a charging apparatus of the type controlling the charging current through detection of the change of voltage of the secondary battery during charging.
2. Description of Related Art
The voltage of a secondary battery such as an Ni-Cd battery or the like rises as the battery is charged, and drops when it is fully charged. Charging apparatus already in the market makes use of this characteristic of the voltage change to control charging current of the secondary batteries. That is, the charging current is reduced or the charging is stopped when the voltage of the secondary battery starts to fall from the maximum point.
An example to control the charging current in the manner as above through detection of the voltage change of the secondary battery is revealed in Japanese Patent Application Laid-Open 103544(1978), U.S. Pat. No. 4,806,840, etc. According to the disclosures, the voltage of the secondary battery is sampled with a predetermined cycle, and two consecutive voltage samples are compared, thereby detecting when the secondary battery is fully charged. In this case, the voltage is sampled by an A/D converter which converts the voltage of the secondary battery from analog signals to digital signals.
Meanwhile, the voltage of a commercial alternating current used as a charging power source varies in some cases due to power ripples or the overlap of external noises. These noises are often overlapped with the loop or node portion appearing every half a cycle of the commercial alternating current of a sine wave. When the secondary battery is charged with the voltage of this charging power source which is overlapped with the above-mentioned noises, the voltage of the secondary battery varies, which leads to the generation of jitters when the voltage is converted to digital signals. In other words, since the voltage of the secondary battery is detected momentarily in the conventional manner, the detection may result in error from the above-described voltage variations of the power source depending on the timing of the sampling.
Further, according to the charging apparatus disclosed in U.S. Pat. No. 4,806,840, the charging is finished when the sampled voltage falls two times in succession. Therefore, if the sampled voltage is varied subsequent to the voltage change of the power source after the secondary battery is fully charged, it is impossible to control the charging.